1. Field of the Invention
This invention relates to heat exchangers and to heat transfer tubes employed therein, in general, and to double-walled heat transfer tubes in particular.
2. Description of the Prior Art
In some solar heating arrangements, heat exchangers may be required between the transfer medium circulated through a solar collector and the distribution medium. Furthermore, the U.S. Department of Housing and Urban Development in HUD Intermediate Minimum Property Standards Supplement, Solar Heating and Domestic Hot Water Systems, Document No. 4930.2, 1977 Edition at Section S-515-9 has set forth specific requirements for heat exchangers when a nonpotable liquid is used as the transfer medium between a solar collector and a potable water supply.
More specifically, one requirement is that the heat exchanger be designed such that a minimum of two walls or interfaces are provided between the nonpotable liquid and the potable water supply.
In one prior art heat exchanger, the toxic heat transfer medium circulates within an outer shell. An intermediary non-toxic heat transfer liquid is employed between the outer shell and a tube circuit carrying potable water. As the toxic heat transfer medium circulates through the shell, the intermediary liquid is heated which in turn heats the potable water supply circulating through the tube.
Another arrangement for providing a double separation between the transfer medium and the potable water supply consists of tubing wrapped around and bonded to a tank. The potable water is heated as it circulates through the coil or tank. One problem with this arrangement is that the tubing coil must be adequately insulated to reduce heat losses.
One problem with both arrangements is that they have relatively low heat transfer coefficients.